1. Field of the Invention
The present invention relates to an optoelectronic method of reading data retained on a magnetic medium and the circuit for performing said method. It is particularly applicable to magneto-optical storages mainly utilized in data processing systems.
2. Description of the Prior Art
It is known that data processing systems essentially comprise a central data processing unit (CPU) and a plurality of data input/output units, generally referred to as "peripheral systems". These systems enable data to be entered into the central processing unit where the data is processed by arithmetic and logic units and are then fed back into the output units after processing. The result of the operations is produced by the central processing unit and is then used and analyzed immediately by the user of the data processing systems, or stored for variable periods of time in storages, such as a "mass or bulk storage".
Among the bulk storages currently in use, the magnetic disc storages are the most frequently employed.
In magnetic disc storages the data is carried by magnetic discs and is retained within concentric circular recording tracks. Usually, a string of magnetic data recorded on a disc track presents itself in the form of a succession of small magnetic domains called "elementary domains" distributed throughout the length of the track and having magnetic inductions with the same modulus and of opposite direction.
The present tendency in the evolution of disc storages is to increase both the longitudinal and radial densities of the data. Longitudinal (or linear) density is defined as the number of data per unit of length measured according to the circumference of a track, while radial density is defined as the number of tracks per unit length measured according to the diameter of a track. Magnetic disc storage of the magneto-optic type represent interesting solutions, because the radial and longitudinal densities obtained are of the order of, respectively, 10,000 tracks per cm (25,000 tracks per inch, or 25,000 tpi) and 10,000 bits per cm (or 25,000 bits per inch, or 25,000 bpi). The word "bit" designates at the same time the binary unit of data which is equal to 1 or 0 and any representation of said information which may, for example, be a magnetic elementary domain retained on a magnetic-disc track or an electrical analog or logic signal. Thus, in magneto-optic storages the dimensions of the elementary magnetic domains are of the order of 1 to 2 microns, and the width of the tracks is of the same order of magnitude.
In magneto-optic storages, writing is performed by devices which are for the most part magnetic, while the reading is performed by an optoelectronic device comprising an assembly of optical means and photoelectronic transducers which convert the light they receive into an electrical signal. In other words, magneto-optic storages are storages where the data is carried by magnetic discs and read by optoelectronic devices.
In magneto-optic storages, the reading mode of the data is based on the principle of interaction of a polarized light with the magnetic state of the layer constituting the magnetic disc, which interaction results in the rotation of the electric-field vector in the plane perpendicular to the direction of propagation (and, thereby, of the magnetic field of the electromagnetic radiation constituting the polarized light).
It will be recalled that an electromagnetic radiation (or, by extension, light) is polarized rectilinearly in the plane where the electric-field vector (and, thereby, the magnetic-induction vector) always keeps the same direction in the plane perpendicular to the direction of propagation of the radiation regardless of the position of said plane in space, and regardless of the instant of observation. The polarization plane is defined as the plane which contains the direction of propagation of the light, and the electric-field vector.
Optoelectronic devices for reading data retained on a magnetic medium that use the interaction principle mentioned above are old in the art. Such a device is, for example, described in French Patent Application No. 81 19543 filed on Oct. 16, 1981 by the applicant and entitled "Optoelectronic Device for Reading Data Retained on a Magnetic Medium" (U.S. Ser. No. 417,904, filed Sept. 14, 1982). The principle of such a device is to send to the surface of the disc a normal, relatively monochromatic, light beam whose diameter is such that it permits observation of a plurality of tracks (of the order of from one dozen to several dozens), and to each track a plurality of magnetic domains, and to project the image of the disc surface, which is illuminated by the light beam, onto a plane where a plurality of photoelectronic transducers are arranged. If the magnetic medium of the magnetic layer, which magnetization is perpendicular (one could also use a magnetic medium whose magnetization is longitudinal, i.e., parallel, to the layer), it will be observed that, following the reflection of the incident beam on said layer and for each magnetic domain, the electric-field vector of the incident light undergoes a rotation in the plane which is normal to the direction of propagation of the light which, by convention is said to be equal to an angle (-.theta.) when the light beam encounters a domain with negative magnetization and which is equal to an angle (+.theta.) when the incident light beam encounters a domain with positive magnetization. This physical phenomemon which has just been described (the interaction of the incident light with the magnetic state of the material which results in a rotation of the magnetic-field vector and of the electric-field vector) is called "Kerr-effect".
Thus, in order to determine the value of a bit, it is sufficient to detect the rotation of the electric-field vector. This is done by means of an "analyzer" formed by a crystal which overrides a direction of propagation of the light and which is placed in such a way that said direction is at right angles to the direction assumed by the electric-field vector of the reflected light when the latter is reflected on a magnetic domain with negative magnetization. Under these conditions, a light with zero intensity is collected at the output of the analyzer. By contrast, when the light is reflected on a magnetic domain with positive magnetization, a light with non-zero intensity is observed at the output of the analyzer. In other words, the domains with negative magnetization will appear in black on the plane where the photoelectronic transducers are arranged, which planes are situated at the output of the analyzer, while the domains with positive magnetization will appear in white. Thus, one sees that the image of the surface of the disc which is lit by the beam with a diameter D which is projected onto the plane, is formed by a group of luminous dark and white spots, depending on the value of the data bits registered on each track of the magnetic disc.
More specifically, the optoelectronic device whose operating principle has been recalled hereinabove comprises:
a source transmitting a polarized light beam sent to the surface of the data medium at a location determined by the latter and whose interaction with the magnetic state of the data medium at said location produces a rotation of the plane of polarization of the light;
means for detecting the angle of rotation of said plane, comprising a light analyzer, and optoelectronic transducers which generate an electric signal whose voltage (or current) is a function of a value of the data retained at said location, and is characterized in that it comprises:
optical means for focussing a light beam with a large field of observation to permit the simultaneous observation of a plurality of tracks and a plurality of data on each track; and
means for projecting the image of the surface of the data medium observed by the objective onto a plane P where the optoelectronic transducers are located.